The present invention relates to chemical mechanical polishing apparatus used in the polishing of semiconductor wafers. More particularly, the present invention relates to an assembly for tensioning a polishing belt on a CMP apparatus to prevent deformation of the belt through prolonged use.
In the fabrication of semiconductor devices from a silicon wafer, a variety of semiconductor processing equipment and tools are utilized. One of these processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shadow trench isolation (STI) layer, inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer, which are frequently used in memory devices. The planarization process is important since it enables the subsequent use of a high-resolution lithographic process to fabricate the next-level circuit. The accuracy of a high resolution lithographic process can be achieved only when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of semiconductor devices.
A global planarization process can be carried out by a technique known as chemical mechanical polishing, or CMP. The process has been widely used on ILD or IMD layers in fabricating modern semiconductor devices. A CMP process is performed by using a rotating platen in combination with a pneumatically-actuated polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica or aluminum.
A polishing pad used on a rotating platen is typically constructed in two layers overlying a platen, with a resilient layer as an outer layer of the pad. The layers are typically made of a polymeric material such as polyurethane and may include a filler for controlling the dimensional stability of the layers. A polishing pad is typically made several times the diameter of a wafer in a conventional rotary CMP, while the wafer is kept off-center on the pad in order to prevent polishing of a non-planar surface onto the wafer. The wafer itself is also rotated during the polishing process to prevent polishing of a tapered profile onto the wafer surface. The axis of rotation of the wafer and the axis of rotation of the pad are deliberately not collinear; however, the two axes must be parallel. It is known that uniformity in wafer polishing by a CMP process is a function of pressure, velocity and concentration of the slurry used.
A CMP process is frequently used in the planarization of an ILD or IMD layer on a semiconductor device. Such layers are typically formed of a dielectric material. A most popular dielectric material for such usage is silicon oxide. In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed is normally between about 5000 A and about 10,000 A. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window-etching or plug-formation difficulties. The CMP process has also been applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing.
Important components used in CMP processes include an automated rotating polishing platen and a wafer holder, which both exert a pressure on the wafer and rotate the wafer independently of the platen. The polishing or removal of surface layers is accomplished by a polishing slurry consisting mainly of colloidal silica suspended in deionixed water or KOH solution. The slurry is frequently fed by an automatic slurry feeding system in order to ensure uniform wetting of the polishing pad and proper delivery and recovery of the slurry. For a high-volume wafer fabrication process, automated wafer loading/unloading and a cassette handler are also included in a CMP apparatus.
As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydrogenation reaction; the formation of hydrogen bonds between the wafer and the slurry; the formation of molecular bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP polishing process is not a mechanical abrasion process of slurry against a wafer surface.
While the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in controlling polishing rates at different locations on a wafer surface. Since the polishing rate applied to a wafer surface is generally proportional to the relative rotational velocity of the polishing pad, the polishing rate at a specific point on the wafer surface depends on the distance from the axis of rotation. In other words, the polishing rate obtained at the edge portion of the wafer that is closest to the rotational axis of the polishing pad is less than the polishing rate obtained at the opposite edge of the wafer. Even though this is compensated for by rotating the wafer surface during the polishing process such that a uniform average polishing rate can be obtained, the wafer surface, in general, is exposed to a variable polishing rate during the CMP process.
Recently, a chemical mechanical polishing method has been developed in which the polishing pad is not moved in a rotational manner but instead, in a linear manner. It is therefore named as a linear chemical mechanical polishing process, in which a polishing pad is moved in a linear manner in relation to a rotating wafer surface. The linear polishing method affords a more uniform polishing rate across a wafer surface throughout a planarization process for the removal of a film layer from the surface of a wafer. One added advantage of the linear CMP system is the simpler construction of the apparatus, and this not only reduces the cost of the apparatus but also reduces the floor space required in a clean room environment.
A typical linear CMP apparatus 10 is shown in FIGS. 1A and 1B. The linear CMP apparatus is utilized for polishing a semiconductor wafer 24, i.e., a silicon wafer in removing a film layer of either an insulating material or a conductive material from the wafer surface. For instance, the film layer to be removed may include insulating materials such as silicon oxide, silicon nitrite or spin-on-glass material or a metal layer such as aluminum, copper or tungsten. Various other materials such as metal alloys or semi-conducting materials such as polysilicon may also be removed.
As shown in FIGS. 1A and 1B, the wafer 24 is mounted on a rotating platform, or wafer holder 18, which rotates at a predetermined speed. The major difference between the conventional linear CMP apparatus 10 and the predecessor CMP apparatus (not illustrated) is that a continuous, or endless, polishing belt 12 is utilized instead of a rotating polishing pad. The polishing belt 12 moves in a linear, rather than rotational, manner in respect to the rotational surface of the wafer 24. The linear polishing belt 12 is mounted in a continuous manner over an idle roller 15 and a drive roller 14, which is driven by a motor (not shown) at a predetermined rotational speed. The rotational motion of the drive roller 14 and idle roller 15 is transformed into a linear motion 26 in respect to the surface of the wafer 24. This is shown in FIG. 1B.
In the conventional linear CMP apparatus 10, one or more polishing pads 30 are adhesively joined to the continuous polishing belt 12 on its outer surface that faces the wafer 24. A polishing assembly 38 is thus formed by the continuous polishing belt 12 and the polishing pad 30 glued or otherwise attached thereto. As shown in FIG. 1A, a plurality of polishing pads 30 are typically utilized and which are frequently supplied in rectangular-shaped pieces with a pressure-sensitive layer coated on the back side.
The wafer platform 18 and the wafer 24 forms an assembly of a wafer carrier 28 on the conventional linear CMP apparatus 10. The wafer 24 is normally held in position by a mechanical retainer, commonly known as a retaining ring 16, as shown in FIG. 1B. The major function of the retaining ring 16 is to fix the wafer 24 in position in the wafer carrier 28 during the linear polishing process and thus, prevent the wafer 24 from moving horizontally as wafer 24 contacts the polishing pad 30. The wafer carrier 28 is normally operated in a rotational mode such that a more uniform polishing on the wafer 24 can be achieved. To further improve the uniformity of linear polishing, a support housing 32 is further utilized to provide support to a support platen 22 during a polishing process. The support platen 22 provides a supporting platform for the underside of the continuous polishing belt 12 to ensure that the polishing pad 30 makes sufficient contact with the surface of the wafer 24 in order to achieve more uniform material removal from the surface layer of the wafer 24. Typically, the wafer carrier 28 is pressed downwardly against the continuous polishing belt 12 and the polishing pad 30 at a predetermined force such that a suitable polishing rate on the surface of the wafer 24 can be obtained. Air pressure is typically further used to push the support platen 22 upwardly against the polishing belt 12 which, in turn, pushes the polishing pad or pads 30 against the wafer 24. A desirable polishing rate on the wafer surface can therefore by obtained by suitably adjusting the downward force on the wafer carrier 28, the upward air pressure against the support platen 22, and the linear speed 26 of the polishing pad 30. A slurry dispenser 20 is further utilized to dispense a slurry solution 34 onto the polishing pad or pads 30. An air cylinder 34 may be connected to the idle roller 15 through a tensioning shaft 36 to facilitate positioning of the idle roller 15 along a horizontal axis and tightening of the polishing belt 12 on the drive roller 14 and idle roller 15.
A common problem associated with the conventional CMP apparatus 10 is that the polishing belt 12 eventually stretches and becomes deformed through prolonged use, as shown in FIG. 2, due to the constant tension exerted on the polishing belt 12 by the support platen 22, wafer holder 18 and idle roller 15. This adversely affects the rate of removal of copper from the semiconductor wafer 24, as well as the removal uniformity, during the CMP process. Consequently, the polishing belt 12 must be frequently replaced, and this requires excessive manpower and costs for preventative maintenance. Accordingly, a device is needed for exerting tension on a polishing belt for a CMP apparatus in order to compensate for enlargement of the belt as a result of prolonged use and maintain proper tension on the belt for effective and uniform polishing of semiconductor wafers.
An object of the present invention is to provide a tensioning assembly for tensioning a polishing belt on a chemical mechanical polisher.
Another object of the present invention is to provide a tensioning assembly which exerts a selected degree of tension on a polishing belt of a chemical mechanical polisher in order to compensate for gradual enlargement and loosening or slackening of the belt resulting from prolonged usage.
Still another object of the present invention is to provide a tensioning assembly which reduces manpower and costs associated with periodic maintenance of chemical mechanical polishers.
Yet another object of the present invention is to provide a tensioning assembly which extends the useful lifetime of a polishing belt in a chemical mechanical polisher.
Still another object of the present invention is to provide a tensioning assembly which contributes to enhanced rates of removal of material from semiconductor wafers by maintaining tension on a polishing belt of a chemical mechanical polishing apparatus.
A still further object of the present invention is to provide a tensioning assembly which facilitates uniform and stable copper removal rates using a polishing belt on a chemical mechanical polisher over prolonged use of the polishing belt.
Yet another object of the present invention is to provide a tensioning assembly for a polishing belt on a chemical mechanical polisher, which tensioning apparatus is capable of applying variable tension on the polishing belt depending upon the slack in the polishing belt in order to achieve uniform and optimum removal of copper or other materials from a semiconductor wafer during a CMP process.
In accordance with these and other objects and advantages, the present invention comprises a tensioning assembly for a polishing belt on a linear chemical mechanical polishing apparatus. The tensioning assembly comprises first and second rollers which are operably engaged by respective air cylinders and exert a selected degree of downward tension on the lower run of the horizontal polishing belt. A third roller biased typically by a spring pushes upwardly on the lower run of the belt between the first and second rollers. Accordingly, the first and second rollers, in conjunction with the third roller, tension the belt on the apparatus to maintain optimum material removal rates and uniformity. The degree of tension exerted on the belt can be varied according to stretching of the belt resulting from prolonged use.